The performance of portable telephone units and similar devices has become very sophisticated, yet the overall size of such devices has become remarkably small. It is very important for electronic parts and components used therein to be small and thin, and to provide complex functions. In such devices, the semiconductor integrated circuits and surface acoustic wave devices (SAW devices) occupy a substantial amount of space therein, among other electronic parts and components. Therefore, downsizing these items is a matter of great importance.
In the SAW device sector, downsizing has been pursued mostly through optimization in the pattern layout of electrodes. However, there is a limitation in this approach.
FIG. 14 shows a typical electrode pattern of a conventional SAW device 200. SAW device 200 includes interdigital transducer electrodes (IDT electrodes) 202A, 202B, 202C, a reflector electrode 203 provided at both sides of the IDT electrodes, a ground pad 204 connected to a grounding electrode terminal 10, an input pad 205 and an output pad 206 provided on a piezoelectric substrate 201. Each of IDT electrodes 202A, 202B and 202C is formed of a comb shaped electrode pair in which a plurality of finger electrodes are coupled to each other. A first part of the each comb shaped electrode pair forming IDT electrodes 202A and 202C located at both ends is connected with separate ground pads 204, while a second part is connected together with common output pad 206. A first part of the comb shaped electrode pair forming the central IDT electrode 202B is connected with input pad 205, while a second part is connected with ground pad 204.
Ground pad 204 is connected with grounding terminal 10 formed on a stem (not shown) by means of a wire lead or other suitable connecting structure.
As shown in FIG. 14, there are three ground pads 204, one input pad 205 and one output pad 206 on the SAW device; totaling five pads, each occupying a certain specific area. An area occupied by a pad is substantially large in relation to that occupied by IDT electrodes 202A, 202B, 202C and reflector electrode 203. The area required for five pads represents the largest factor against device downsizing. In addition, since respective ground pads 204 are grounded (with grounding electrode terminals) separately, the ground level is not low enough, and is not stable, thereby leading to deteriorated filter characteristics.
In a SAW device, IDT electrodes have been reduced in size by shrinking the electrode pattern and optimizing the layout. On the other hand, the reduction of pad area remains very difficult because ground pads, input pads and output pads for connection with external terminals require a certain minimum area if a certain connecting strength with a wire lead and reliability in electrical connection are to be guaranteed. This means that further reduction in the pad area is not an easy task. Instead, a reduction in the number of pads is more effective for downsizing a SAW device.
Other methods have been proposed to reduce the device size of a SAW device such as eliminating a stage-to-stage coupling capacitance section which used to be added for improving the cutoff characteristics of a passing band and increasing attenuation level, or an inductance section. For example, Japanese Patent Laid-Open Application No. H10-224179 discloses a method of eliminating the stage-to-stage inductance, in a device formed of primary-tertiary serial coupling double mode surface acoustic wave filters consisting of three IDT electrodes and two reflectors disposed at both sides of the IDT electrodes on a piezoelectric substrate, wherein the filters are serially connected in multi-stages. Namely, the stage-to-stage inductance is made to be approximately zero in terms of capacitance by making the electrode pair counts to be different between the IDT electrodes located at both sides, among the three IDT electrodes; and the end-to-end spaces from the central IDT electrode to the IDT electrodes on either side was established to be approximately 5λ/4 for one of the spaces, and the other space to be different from that of the first.
However, even if the stage-to-stage coupling capacitance section or inductance section is eliminated, like in the above example, there is a limitation in the downsizing. Furthermore, since connection to grounding terminals from respective pads is made separately, the ground level is not sufficiently low, and not stable. Thus, the problem of deterioration in the filter characteristics remains unsolved, like in the first example discussed above.